I won't explain every detail in the beginning, you can get to the state in the next image several different ways. For example like this: polygonal box modeling. (This isn't meant to be a step-by-step cloning; it's more of a general workflow thing.)
[Above]

This is the model I started with for this session - a simple model several years old that I'd previously used as the 0-level of a hierarchical SubD surface. But I wanted to rebuild it without the hierarchy (mainly because I plan to render it in Renderman one day). [Below]

Say we're going for realism. Well in that case this isn't working. It needs more detail, and for that we need reference. Also, the topology is bad. (I could make it better without reference, but I knew I'd need it before the end so I made it the first order of business.)

The best reference is without a doubt the actual human body, and I did use my own foot to some extent, but good photographs do have certain advantages over the living body (like for instance, my foot is really ugly). I used reference images from Peter Levius great site www.3d.sk - I found about 4 different women's feet there, from several angles, quite close up, flat on the floor, which was exactly what I needed. (I can't show all of them, for copyright reasons.)

To those who doubt the usefulness of a traditional art education to a 3d modeler, I hope the following may help to sway your opinion a little. First I sketched the major shapes I could see, then I sketched a preliminary topology. There's also a cross-section showing the spacing of vertical edges around the back of the ankle.

From here I started adding and deleting edges to the old model. On the left, the red lines indicate edges being added, and on the right, the highlighted edges (orange) are about to be deleted.